1. Topics discussed in this section:
4-2 ANALOG-TO-DIGITAL CONVERSION
A digital signal is superior to an analog signal because it is more robust to noise and can easily be recovered, corrected and amplified. For this reason, the tendency today is to change an analog signal to digital data. In this section we describe two techniques, pulse code modulation and delta modulation.
Topics discussed in this section:
Pulse Code Modulation (PCM)
Delta Modulation (DM)

2. PCM PCM consists of three steps to digitize an analog signal:
Sampling
Quantization
Binary encoding
Before we sample, we have to filter the signal to limit the maximum frequency of the signal as it affects the sampling rate.
Filtering should ensure that we do not distort the signal, ie remove high frequency components that affect the signal shape.

3. Figure 4.21 Components of PCM encoder

4. Sampling Analog signal is sampled every TS secs.
Ts is referred to as the sampling interval.
fs = 1/Ts is called the sampling rate or sampling frequency.
There are 3 sampling methods:
Ideal - an impulse at each sampling instant
Natural - a pulse of short width with varying amplitude
Flattop - sample and hold, like natural but with single amplitude value
The process is referred to as pulse amplitude modulation PAM and the outcome is a signal with analog (non integer) values

5. Figure 4.22 Three different sampling methods for PCM

6. According to the Nyquist theorem, the sampling rate must be
Note
According to the Nyquist theorem, the sampling rate must be
at least 2 times the highest frequency contained in the signal.

7. Figure 4.23 Nyquist sampling rate for low-pass and bandpass signals

8. Figure 4.24 Recovery of a sampled sine wave for different sampling rates

9. Figure 4.25 Sampling of a clock with only one hand

10. Quantization
Sampling results in a series of pulses of varying amplitude values ranging between two limits: a min and a max.
The amplitude values are infinite between the two limits.
We need to map the infinite amplitude values onto a finite set of known values.
This is achieved by dividing the distance between min and max into L zones, each of height 
 = (max - min)/L

11. Quantization Levels
The midpoint of each zone is assigned a value from 0 to L-1 (resulting in L values)
Each sample falling in a zone is then approximated to the value of the midpoint.

12. Quantization Zones
Assume we have a voltage signal with amplitutes Vmin=-20V and Vmax=+20V.
We want to use L=8 quantization levels.
Zone width = ( )/8 = 5
The 8 zones are: -20 to -15, -15 to -10, -10 to -5, -5 to 0, 0 to +5, +5 to +10, +10 to +15, +15 to +20
The midpoints are: -17.5, -12.5, -7.5, -2.5, 2.5, 7.5, 12.5, 17.5

13. Assigning Codes to Zones
Each zone is then assigned a binary code.
The number of bits required to encode the zones, or the number of bits per sample as it is commonly referred to, is obtained as follows:
nb = log2 L
Given our example, nb = 3
The 8 zone (or level) codes are therefore: 000, 001, 010, 011, 100, 101, 110, and 111
Assigning codes to zones:
000 will refer to zone -20 to -15
001 to zone -15 to -10, etc.

14. Figure 4.26 Quantization and encoding of a sampled signal

15. Quantization Error
When a signal is quantized, we introduce an error - the coded signal is an approximation of the actual amplitude value.
The difference between actual and coded value (midpoint) is referred to as the quantization error.
The more zones, the smaller  which results in smaller errors.
BUT, the more zones the more bits required to encode the samples -> higher bit rate

16. Quantization Error and SNQR
Signals with lower amplitude values will suffer more from quantization error as the error range: /2, is fixed for all signal levels.
Non linear quantization is used to alleviate this problem. Goal is to keep SNQR fixed for all sample values.
Two approaches:
The quantization levels follow a logarithmic curve. Smaller ’s at lower amplitudes and larger’s at higher amplitudes.
Companding: The sample values are compressed at the sender into logarithmic zones, and then expanded at the receiver. The zones are fixed in height.

17. Bit rate and bandwidth requirements of PCM
The bit rate of a PCM signal can be calculated form the number of bits per sample x the sampling rate
Bit rate = nb x fs
The bandwidth required to transmit this signal depends on the type of line encoding used. Refer to previous section for discussion and formulas.
A digitized signal will always need more bandwidth than the original analog signal. Price we pay for robustness and other features of digital transmission.

18. PCM Decoder
To recover an analog signal from a digitized signal we follow the following steps:
We use a hold circuit that holds the amplitude value of a pulse till the next pulse arrives.
We pass this signal through a low pass filter with a cutoff frequency that is equal to the highest frequency in the pre-sampled signal.
The higher the value of L, the less distorted a signal is recovered.

19. Figure 4.27 Components of a PCM decoder

20. Delta Modulation
This scheme sends only the difference between pulses, if the pulse at time tn+1 is higher in amplitude value than the pulse at time tn, then a single bit, say a “1”, is used to indicate the positive value.
If the pulse is lower in value, resulting in a negative value, a “0” is used.
This scheme works well for small changes in signal values between samples.
If changes in amplitude are large, this will result in large errors.

21. Figure 4.28 The process of delta modulation

22. Figure 4.29 Delta modulation components

23. Figure 4.30 Delta demodulation components

24. Delta PCM (DPCM)
Instead of using one bit to indicate positive and negative differences, we can use more bits -> quantization of the difference.
Each bit code is used to represent the value of the difference.
The more bits the more levels -> the higher the accuracy.

25. Topics discussed in this section:
4-3 TRANSMISSION MODES
The transmission of binary data across a link can be accomplished in either parallel or serial mode. In parallel mode, multiple bits are sent with each clock tick. In serial mode, 1 bit is sent with each clock tick. While there is only one way to send parallel data, there are three subclasses of serial transmission: asynchronous, synchronous, and isochronous.
Topics discussed in this section:
Parallel Transmission
Serial Transmission

26. Figure 4.31 Data transmission and modes

27. Figure 4.32 Parallel transmission

28. Figure 4.33 Serial transmission

29. Note
In asynchronous transmission, we send 1 start bit (0) at the beginning and 1 or more stop bits (1s) at the end of each byte. There may be a gap between each byte.

30. Asynchronous here means “asynchronous at the byte level,”
Note
Asynchronous here means “asynchronous at the byte level,”
but the bits are still synchronized; their durations are the same.

31. Figure 4.34 Asynchronous transmission

32. Note
In synchronous transmission, we send bits one after another without start or stop bits or gaps. It is the responsibility of the receiver to group the bits. The bits are usually sent as bytes and many bytes are grouped in a frame. A frame is identified with a start and an end byte.

33. Figure 4.35 Synchronous transmission

34. Isochronous
In isochronous transmission we cannot have uneven gaps between frames.
Transmission of bits is fixed with equal gaps.